1. Field of the Invention
The present invention relates to an optical recording medium and a stamper for manufacturing the optical recording medium which provides conditions under which a playback signal is stabilized in the optical recording medium which records information using lands and grooves and has a shallow & deep structure with one groove different in depth from the other groove.
2. Description of the Related Art
In writable optical disks such as a magnetooptical disk or a phase change recording disk, a groove is typically formed along a recording track on a disk substrate. The groove here refers to a so-called guide groove that is formed along a recording track and is mainly intended to assist tracking servo. A region present between grooves is referred to as a land.
To achieve a high density of recording, increasing a track pitch is as important as increasing a line density. The following methods are available to increase the track pitch in a recording mode in which signals are recorded in both the land and the groove: a land & groove method and a shallow & deep groove method which has been proposed in Japanese Unexamined Patent Application Publication No. 11-296910 which is assigned to the same assignee of the present invention. The shallow & deep method employs a pair of two spiralling grooves, one shallow and the other deep, with a land interposed therebetween.
Discussed below is the shallow & deep groove method having the grooves varied and recording a signal on the land thereof as disclosed in Japanese Unexamined Patent Application Publication No. 11-296910.
In conventional optical disks, the depth and the width of the grooves of adjacent tracks remain unchanged. If the track pitch is made fine in this structure, the spatial frequency of the track exceeds an MTF (Modulation Transfer Function), and no tracking signal is generated. Although there is the potential that the recording density will be increased in recording and playback characteristics, the tracking pitch is subject to a limitation, in practice, because of an inability to track. In the shallow & deep groove method, the depth of the grooves is alternately changed. In this way, a frequency component half the track pitch is generated, and a tracking error signal is obtained. If one track is split into two, the two split tracks become mirror symmetrical to each other, thereby helping regulate recording characteristics. In contrast, in the land & groove method, two different portions, namely, the land and groove, record a signal, and there occurs variations in recording characteristics. The shallow & deep groove method has this advantage over the land & groove method.
In the shallow & deep groove method, the tracking error signal has the period of two tracks. A difference signal and a sum signal in this method are thus different from those in the normal tracking method. The difference signal and the sum signal become different in the magnitudes thereof depending on the depths of the two grooves. The sum signal refers to a sum of a first optically detected output signal resulting from the reflection and diffraction of a light beam and a second optically detected output signal resulting from the reflection and diffraction of the light beam. The difference signal is also referred to as a push-pull signal. The sum signal is also referred to as a cross-track signal.
To perform tracking servo using the push-pull signal in the current high-recording density optical disk, a push-pull signal amplitude ratio needs to be 0.15 or greater. To stably seek a track using the cross-track signal, the cross-track signal needs to be 0.06 or more. The push-pull signal amplitude ratio is a ratio of an AC component of the push-pull signal to a mirror reflected component, namely, the maximum value of the sum signal on a mirror surface. The cross-track signal amplitude ratio is a ratio of an AC component of the cross-track signal to a mirror reflected component of the cross-track signal.
The shallow groove as the first groove in a shallow & deep groove optical disk is set to be deeper than that disclosed in Japanese Unexamined Patent Application Publication No. 11-296910, and the second groove is set to be even deeper. With this arrangement, a higher density recording type optical disk is contemplated.
The shallow and deep groove method may be implemented in an optical disk of the type that a recording mark is detected through the Domain Wall Displacement Detection (DWDD). The DWDD is a technique to read a recording mark smaller than an optical spot during playback by expanding a magnetic domain by means of thermal distributions induced by the optical spot. Since the DWDD allows the edge of each mark to be neatly detected, it is appropriate for use in replaying data from a magnetooptical disk that employs a mark edge recording method.
It is said that the grooves need to be deeper than a certain depth, for example, 100 nm in the DWDD when an optical disk of the type that replays a recorded signal through the DWDD is combined with the shallow & deep groove method. In this case, the first groove must be 100 nm, and the second groove must be deeper than that.